A wearer may be prescribed a positive or negative optical power correction. For presbyopic wearers, the value of the power correction is different for far vision and near vision, due to the difficulties of accommodation in near vision. The prescription thus comprises a far-vision power value and an addition representing the power increment between far vision and near vision. The addition is qualified as prescribed addition. Ophthalmic lenses suitable for presbyopic wearers are multifocal lenses, the most suitable being progressive multifocal lenses.
The ophthalmic prescription can include a prescribed astigmatism. Such a prescription is produced by the ophthalmologist in the form of a pair formed by an axis value (in degrees) and an amplitude value (in diopters). The amplitude value represents the difference between minimal and maximal power in a given direction which enables to correct the visual defect of a wearer. According to the chosen convention, the axis represents the orientation of one of two powers with relation to a reference axis and in the sense of rotation chosen. Usually, the TABO convention is used. In this convention, the reference axis is horizontal and the sense of rotation is anticlockwise for each eye, when looking to the wearer. An axis value of +45° therefore represents an axis oriented obliquely, which when looking to the wearer, extends from the quadrant located up on the right to the quadrant located down on the left. Such an astigmatism prescription is measured on the wearer looking in far vision. The term <<astigmatism>> is used to designate the pair (amplitude, angle); despite this use not being strictly correct, this term is also used to refer to the amplitude of the astigmatism. The person skilled in the art can understand from the context which meaning is to be considered. It is also known for the person skilled in the art that the prescribed power and astigmatism of a wearer are usually called sphere SPH, cylinder CYL and axis. FIG. 1 is a schematic illustration of the prescription expressed in TABO referential desired for the left eye of a wearer. The axis of the prescription (65° here) gives the direction of the smallest power which is, in this case, 3.50 δ whereas the highest power is along the direction which is perpendicular to the axis of the prescription and its value corresponds to +3.50 δ+0.25 δ=3.75 δ. The mean power (also called the mean sphere SM) is the arithmetical average of the smallest power and the highest power and is equal to 3.625 δ.
As explained above, the most suitable lenses for presbyopic wearers are progressive multifocal lenses. However, such lenses induce optical defects that must be minimised in order to satisfy the wearer. When an image perceived by a wearer is formed through a lens, several phenomena degrading the imaging performances of a lens occur. Power defect, astigmatism defect and high order aberrations are example of optical defects which impact the optical quality of the image, then reducing its sharpness and its contrast. The optical defects also modify the appearance of the object perceived by the wearer. Indeed, an object may appear distorted (the shape of the image is modified) and/or delocalized compared to the object.
When designing a progressive multifocal lens, it is therefore sought to reduce as much as possible the optical defects even tough it is not possible to cancel them completely because of the power increment. Thus, it is also sought to spread the defects in such a way that the wearer's vision is the least affected by the remaining optical defects.
The person skilled in the art knows how to compensate for optical defects which comprise among others the power defect and astigmatism defect as described in EP-A-0,990,939, U.S. Pat. No. 5,270,746 (EP-A-0,461,624) and WO-A-98 12590. The lens designer has to handle two contradicting constraints when compensating the optical defects. On the one hand, he needs to design large central zones to provide the wearer with comfortable vision, when reading for instance. This can be done by pushing away the optical defects in lateral zones of the vision field thereby producing important gradients in the periphery of the vision field which impact dynamic vision. On the other hand, the designer needs to limit the gradients in the periphery of the vision field to improve dynamic vision; this being detrimental to the size of the central vision zone. Known methods oblige to a compromise between central and peripheral vision performances.
Moreover, the above-mentioned methods do only consider optical criteria which first of all improve or degrade the sharpness of the image perceived by the wearer. For instance, criteria of power, astigmatism and higher order of aberration are dealt with. The lens designer will make a compromise among those criteria to limit distortion of the image perceived through the lens. Thereby, the lenses are typically a compromise between sharpness and image deformation.